Abstract
This paper is the second of a two-part series that investigates passive
buoyant tracers in the ocean surface boundary layer (OSBL). The first
part examines the influence of equilibrium wind-waves on vertical tracer
distributions, based on large eddy simulations (LESs) of the
wave-averaged Navier-Stokes equation. Motivated by observations of
buoyant microplastic marine debris (MPMD), this study applies the LES
model and the parametric one-dimensional column model from part one to
examine the vertical distributions of MPMD. MPMD is widely distributed
in vast regions of the subtropical gyres and has emerged as a major open
ocean pollutant whose distribution is subject to upper ocean turbulence.
The models capture shear-driven turbulence, Langmuir turbulence (LT),
and enhanced turbulent kinetic energy input due to breaking waves (BWs).
Model results are only consistent with observations of MPMD profiles and
the relationship between surface concentrations and wind speed if LT
effects are included. Neither BW nor shear-driven turbulence is capable
of deeply submerging MPMD, suggesting that the observed vertical MPMD
distributions are a characteristic signature of wave-driven LT. Thus,
this study demonstrates that LT substantially increases turbulent
transport in the OSBL, resulting in deep submergence of buoyant tracers.
The parametric model is applied to 11 years of observations in the North
Atlantic and North Pacific subtropical gyres to show that surface
measurements substantially underestimate MPMD concentrations by a factor
of 3–13.